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1. Field of the Invention
The present invention relates to bus interfaces, and in particular, to a system and method for providing short circuit protection for a Universal Serial Bus (USB) interface.
2. Description of the Related Art
The Universal Serial Bus (USB) is a bi-directional, isochronous, dynamically attachable serial interface providing two wire point to point signaling. Signals are differentially driven at a bit rate of 12 megabits per second.
An exemplary Universal Serial Bus interface is shown in FIG. 1. As shown, the interface includes a USB transceiver 102, typically embodied as an integrated circuit. At least four contacts are provided at a chip boundary 104. These are VDD (5 V) 106, VSS (0 V) 112, and the data ports DPLS 108 and DMNS 110 (0-3.3 V).
A short circuit of DPLS or DMNS to VDD or VSS while the bus is being driven could damage the transceiver. Accordingly, the Universal Serial Bus Specification Version 1.1 requires that the transceiver connected to DPLS and DMNS must resist a short circuit of DPLS and DMNS to VDD or VSS.
These and other drawbacks in the prior art are overcome in large part by transceiver shortcut protection according to the present invention. A Universal Serial Bus interface according to one implementation includes a USB device controller coupled by way of a shortcut protection circuit to a USB transceiver. The shortcut protection circuit compares the driven DPLS and DMNS signals from the USB transceiver with TxDPLS and TxDMNS, the transmitted signals from the USB device controller. If a difference is detected between TxDPLS and DPLS, or between TxDMNS and DMNS, the transmission is disabled.
A shortcut protection unit according to one implementation includes a comparison circuit and a disable circuit. The comparison circuit is embodied as a state machine for periodically comparing the TxDPLS and DPLS signals, and the TxDMNS and DMNS signals. If different signal levels are detected, then the state machine provides an output to the disable circuit. In turn, the disable circuit turns off the USB transceiver""s transmit enable.